1. Field of the Invention
This invention relates to a dental hand-piece for tooth cutting operation and more particularly to a hand-piece with an air turbine wherein a turbine rotary body rotating normally at about 200,000 to 500,000 r.p.m. is supported by a shaft bearing within a head casing, and is equipped with a plurality of turbine blades on its circumferential surface and a novel chucking means for a cutting tool spindle in its interior.
2. Prior Art
Dental hand-pieces with an air turbine are usually divided broadly into two categories: "air journal type" and "ball bearing type". The former is permitted to rotate at the high speed of about 200,000 to 500,000 r.p.m. normally on the ground that the friction resistance of its shaft bearing part here is infinitesimal, while the latter rotates at the low or middle speed of about 100,000 to 400,000 r.p.m. and nothing more, resulting from the fact that the same resistance increases here in one degree or another.
These types of hand-pieces incorporate in general a chucking means comprising a collet chuck with a plurality of tapered chuck pawls and a clamping member convergently clamping from the outside the chuck pawls into the inside of a turbine rotary body in such manner that the chucking means will be placed in a concentrical position with relation to the rotary body. Hereby a cutting tool spindle appears to be tightly held by the chucking means so as not to slip off. Be that as it may, there is not without any problem here. That is, it is only when the turbine rotary body is stopping that the cutting tool spindle can be firmly held in these chucking means. It becomes difficult, on the contrary, for the cutting tool spindle to be held as thoroughly and steadily as required because the holding force of the centripetal direction is weakened at the time of the high-speed rotation of the rotary body during the tooth cutting operation by the action of a great centrifugal force upon the chuck pawls of the collet chuck. Such circumstances have a very serious influence upon the dental treatment from the viewpoint of safety. Furthermore, in this type of chucking means, the fastening and unfastening operation of the clamping member is required to be done by the hand of the operator whenever the cutting tool will be replaced; this is a very annoying job.
In order to make the replacement of the cutting tool facile, another kind of hand-piece designed so as to be able to do the loading and unloading operation of the cutting tool at one touch has been developed adopting what is called "an elastic chuck" made of synthetic resin or rubber, in place of the above-mentioned chucking means. However, even with this new chucking means still remains unsolved the embarrassing problem that the centrifugal force acting on the elastic chuck leads to the reduction of the holding force of the centripetal direction while working.
As understood from the above description, it can be said that there is not developed as yet such an excellent air turbine hand-piece for dental use as to be equipped with a chucking means which is able to surely and steadily hold the cutting tool spindle at the time of the high-speed rotation of the turbine rotary body and which is highly convenient for the facile replacement of the cutting tool. Hence, there are loud cries among the persons concerned for the hoped-for development of an excellent hand-piece such as mentioned just now.